The term "thick film resistor" as used in the electronic industry refers to the method used to fabricate a resistor rather than the relative thickness of the individual layers of material which comprise the resistor. In the manufacture of thick film resistors, especially formulated pastes are typically applied and fired in a predetermined sequence on a ceramic substrate. These pastes include conductor pastes and resistor pastes. The conductor pastes are typically comprised of fine particles of a highly conductive metal, such as gold or a gold alloy, and an organic carrier. When a layer of a conductive paste is fired, the organic carrier is burned off and a conductive metal layer is formed. The resistor pastes are typically comprised of a mixture of two principal components, a dielectric glass frit and an electrically conductive powder. When a layer of resistor paste is fired, the glass frit is fused and a layer of resistor material is obtained having a glassy matrix with conductive particles distributed through the matrix. The layer of resistor material will have an ohmic value which is dependent to a large extent on the composition and distribution of the conductive particles in the resistor layer.
When making a conventional thick film resistor, the conductive paste is initially printed on the ceramic substrate in a predetermined pattern and fired to form a pair of spaced apart terminals. Thereafter usually a single layer of a resistor paste is applied between and over a portion of the previously formed terminals and the layer of resistor paste is fired to form a fused resistor body.
The selection of a particular resistor paste for use in the manufacture of a thick film resistor is determined by a number of interrelated factors, such as the ohmic value desired for the completed resistor; the frequency of the current at which the resistor will be exposed; the temperature at which the resistor is expected to operate; and the average power and maximum peak power surges the resistor is anticipated to encounter in use.
The effective ohmic value of a resistor layer is determined to a large extent by the frequency at which the resistor is used. If the frequency is in the RF range, power fed to the resistor will travel almost exclusively along the skin of the resistor layer rather than through the bulk of the resistor body. For this reason, when selecting a resistor material for RF applications, the resistance of the surface area, commonly referred to as the sheet resistance value, is used as a guide in the selection of the resistor material since the sheet resistance of a layer of a fired resistor paste will generally be closely related to the resistance of the final thick film resistor.
The effective ohmic value of a resistor is also highly dependent on the temperature at which the resistor is employed. With most resistor materials, the higher the temperature at which the resistor is operated, the higher the resistance will be. Since it is generally desired to have a constant resistance, it is preferable that the resistors operate at a selected relatively low temperature. Excessively high operating temperatures are a serious problem because the excessively high temperatures can cause electrical breakdown of the resistor body and result in arcing across the resistor.
To fabricate conventional thick film resistors capable of handling relatively high power in the RF range, it is common practice to increase the surface area of the resistor by widening and/or lengthening the resistor. The increased area of the resistor obtained thereby can result in greater dissipation of heat, allowing the resistor to operate at a lower temperature. This approach, while effective in certain applications, has not been proven to be entirely satisfactory however, particularly in high power microelectronic applications, in that the amount that the area of the resistor has to be increased to be effective in maintaining relatively low temperatures can often result in an excessively large area resistor which is unsuitable for microelectronic applications.
An additional cause of failure of resistors, particularly those employed in high power applications, is excessively high current density at various portions of the resistor. One of the areas of a resistor where excessively high current densities are commonly encountered is at the junction of the terminal with the body of the resistor material.
Various suggestions have been made to improve the performance of thick film resistors. One especially notable suggestion was made by Landry et al. in U.S. Pat. No. 4,245,210 entitled "Thick Film Resistor Element And Method Of Fabricating." Landry et al., suggest using three or more distinct overlying layers of resistive material with the bottommost layer being in electrical contact with the terminals of the resistor. The Landry et al. thick film resistor was a significant advance in the resistor art in that it resulted in a resistor which was substantially more compact than the equivalent conventional single layer resistors that were previously employed. The Landry et al. resistor has not, however, proven to be completely satisfactory, especially in high power resistor applications where extremely high peak power loads are encountered and large amounts of heat are produced because Landry et al.'s resistors tend to break down under these conditions.
What would be highly desirable for high power resistor applications is a compact thick film resistor capable of dissipating substantial amounts of heat and reducing operating current densities.